Electrically powered automobiles are vehicles that do not depend on internal combustion engines for propulsive power, but rather on relatively large electric traction batteries. The traction battery of an electric automobile is engaged with an electric traction motor for propelling the automobile, and the traction battery is rechargeable to permit repeated use of the traction battery.
The skilled artisan will appreciate that a traction battery must have a relatively large capacity, and must deliver a relatively large amount of power, compared to a conventional 12 volt automobile storage battery. The skilled artisan will further appreciate that, because power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by traction batteries necessarily mean that higher electrical voltages will be present in electric automobiles than in automobiles powered by fossil fuels, which typically require only a comparatively low power, low voltage storage battery for energizing auxiliary loads when the internal combustion engine is not operating.
Because it doesn't require the combustion of fossil fuels, an electric automobile produces little or no environmentally harmful emissions, in contrast to an automobile powered by fossil fuel. For this reason, electric automobiles may become increasingly attractive alternatives to fossil fuel powered cars. Nonetheless, as implied by the discussion above, because of the high voltage requirements of its traction battery an electric automobile raises significant electrical safety
More particularly, equipment damage, as well as personal electric shock, which arises from unwanted electric current flow outside of the intended electric circuit flow can have graver consequences when the shock is caused by contact with a high voltage traction battery system, as compared to a conventional, relatively low voltage automotive storage battery system. To reduce the likelihood of such shock, many traction battery systems are not grounded to the automobile chassis, in contrast to conventional automotive storage battery systems. Instead, traction battery systems have a closed loop return path, so that the "ground" of the system (i.e., the electrical current return loop) is isolated from the chassis of the electric car. Such a system is referred to as a "floating ground" system.
The safety advantage inherent in a floating ground traction battery system is that a single fault that creates an electric current path from the system to chassis ground will not result in current flow through the fault medium. This is because, in a floating ground system, a complete current path which is otherwise required for electrical current to flow (and, hence, which is otherwise required to produce electric shock) is not established by only a single fault to chassis ground. Instead, two faults, both of which must effectively short the traction battery system to chassis ground, are required to thereby complete an electric current path before electric shock becomes possible.
As recognized by the present invention, while a single electric fault, or short, between a traction battery system and chassis ground will not cause unwanted current flow, nonetheless it would be advantageous to detect such a fault as soon as it occurs, so that the fault may be corrected before a second fault develops. As further recognized by the present invention, while ground fault detection systems exist, existing ground fault detection systems have certain drawbacks when used in traction battery applications.
For example, the so-called ground fault circuit interrupt (GFCI) system, which measures a current differential between a supply current lead and a return current lead to detect a fault, cannot easily be used for traction battery applications for several reasons. First, more than a single component may function as a power source, depending on the mode of operation of the traction battery system, and thus multiple GFCI sensors would be required. Specifically, the traction battery itself functions as the power source during automobile operation, but the traction motor assumes the power source function during regenerative braking, thus requiring GFCI sensors near both the traction battery and traction motor. Further, when the traction battery is being recharged the battery charger functions as the power source. Consequently, multiple GFCI sensors would be required, and even with multiple sensors, any fault which occurred upstream of a sensor (i.e., between a sensor and a power source) would remain undetected by a GFCI system.
Another existing fault detection system is the resistor bridge circuit, which requires the creation of a short to chassis ground through which test current can flow. As alluded to above, the safety drawbacks inherent in the creation of such a short to chassis ground may be acceptable for some applications, but are unacceptable for high voltage applications. Moreover, the flow of test current through a resistor bridge circuit would tend to unacceptably cause a high current drain on the traction battery.
Still further, voltage variations in a traction battery are considerable, and a fault detection system which has a sensitivity appropriate for monitoring a fully charged traction battery may not be appropriate for monitoring a nearly depleted traction battery, and vice versa.
Accordingly, it is an object of the present invention to provide a system for detecting faults in automobile traction battery systems which is safe and which does not unduly cause traction battery drain. Another object of the present invention is to provide a system for detecting faults in automobile traction battery systems which minimizes the number of fault sensors that must be used. Yet another object of the present invention is to provide a system for detecting faults in automobile traction battery systems which has a sensitivity that changes appropriately with traction battery voltage. Still another object of the present invention is to provide a system for detecting faults in automobile traction battery systems which is easy to use and cost-effective.